Tactical squads need to be built to fulfill a role, typically a shooting role, and I generally keep them as cheap as possible (within reason).
10 man strong is never a bad idea, as you want your scoring unit to be able to last all game and not be whittled down to one or two models when exposed to some heavier firepower. This also allows you to combat squad the unit into two five man fire teams as well. The counter to this is if you're fielding multiple small unit style lists (many 5 man teams). Which has it's own advantages, but for the purposes of this discussion, I'll stick with 10 man squad sizes.
You want a heavy weapon, and the two cheapest are the Heavy Bolter, or Multi melta. Both are good options, though the latter is woefully short ranged for my tastes. The option I would consider would be the Missile Launcher, as it combines very good range, with the option to fire one high strength, lower
AP shot, or a small blast template as well for clusters of enemies (Tyranid broods o tau fire warriors clustered in a building don't like blasts in my experience). This costs just 5 points more than the Heavy Bolter or Multimelta, and later on if you want to spend the extra points you can also throw flak missiles on the missile launcher so you have a means to threaten flying targets. If you want a dedicated anti-tank weapon, you can't beat a Lascannon. It's pricey, but worth it if you need to crack armor open more reliably with
AP:2 and high strength. In order of use, I would suggest Missile Launcher first (with flak missile upgrade if you're fighting something with flyers), then Lascannon or Heavy Bolter (equally good, depending on points and/or what you want the squad to focus on: Anti vehicle/hard-target or anti-infantry).
Next comes the special weapon, of these options I generally always defer to the Plasma Gun, it's range gels exactly with the Bolters over the rest of the squad, and it has good ability to hurt high toughness & low armor save targets such as big monstrous creatures with 3+/2+ armor saves. It can overheat, yes, but with a 3+ save odds are good you won't cook yourself too often when you do overheat. Gravguns are also interesting options, and I'd say look at them/play test them, but their shorter range may be more of a detriment vs. the good old plasma gun. Being Salvo doesn't suck though, but even still, I think you'll find for general use, the plasma will do the job and if something is close enough, reliably being able to double-tap with it is always nice. Melta guns and Flamers are both viable options too, if you're looking to shave points, but again the range issue rears its head with these weapons more than the Gravguns. If you KNOW the squad is going to be danger-close/in rapid fire range, then I'd consider a flamer or Meltagun, depending on what you want the squad to hunt, but only if you intend to pile them out of a drop pod next to a target. Otherwise that marine will not be able to shoot very often if ever, and why spend the points if the model can't shoot at the target the rest of the squad is targeting? In order of usefulness I'd rate the special weapons at: Plasma Gun, Grav Gun, Flamer, Meltagun.
Lastly, your Sergeant. I like the thought of upgrading your Sergeant to a Veteran for the +1 leadership, however that's not critical, and only do it if you've got the spare points for it. If you're taking a lot of leadership/pinning tests, then you may want to consider paying for that upgrade. However, if you do, then you should try to limit his wargear somewhat. My rule of thumb is never spend too much more on wargear for a 1 wound character. If the character costs me say, 24 points, I never spend more than 20 to 25 points on wargear in total. I personally like making sure a shooty tactical squad is as shooty as possible. That's their role, to stand on an objective or move toward an objective and constantly be putting bolter rounds toward a target. As such, I generally will only ever put a Combiplasma/melta on a Sergeant, depending on the squad's role. If and only if I've got some extra points laying around, I might consider slapping a power weapon of some type, or Meltabombs, on a Sergeant.
My argument against power weapons on a character is that it feels like wasted points if I never get the squad in close combat, or if the character dies before making it to close combat and not using that weapon I paid for. I can typically use the one-shot special weapon round from a combiplasma or combimelta though, typically to good effect.
It's your squad, and if you want the sergeant to be a jack of all trades, I can totally respect that, though. Here's how I would build your squad were I you, based on the above information.
Tactical Squad:
7x Bolter Marines
1x Heavy Weapon Marine w/ Missile Launcher (Or, Heavy Bolter)
1x Special Weapon Marine w/ Plasma Gun
1x Sergeant (Or Veteran Sgt.) w/ Combiplasma & Meltabomb
Altenate Sergeant/Veteran Sergeant builds: Combiplasma & Power Weapon (Maul or Sword) which will run the squad 10 points more expensive overall.
That gives you a long range missile shot that can either do damage to armor/hard targets or put a small blast on a small group of clustered soft targets, as well as a good volume of bolt shots with plasma helping, and if you need it, a second plasma shot as well on one shooting phase. Also, being able to rapid-fire four plasma shots on a close target is also a nice way to help deal with something when needed (Sergeant using the combiplasma shot to rapid fire 2 rounds at 12" with the squad's plasma gun rapid firing also).
That's my input on the squad, best of luck and welcome to the hobby

Hope your squad does you proud in the future!
Take it easy for now.
-Red__Thirst-